If something has a processor and some sort of storage or networking component, odds are it can run Linux. According to a story on Boing Boing Gadgets, that even applies to Amazon's Kindle.
A hacker by the name of Jesse Vincent reportedly got the ARM release of Ubuntu Linux 9.04 running on the e-book reader. The article includes a photo that shows the second-gen Kindle running "xdaliclock in front of an xterm with the remains of a 'top' command and a few mildly embarrassing typos." Getting Linux onto the device wasn't too hard, Vincent suggests:
To open up the Kindle, I used the USB networking debug mode Amazon left hanging around when they first shipped the Kindle 2, a statically linked telnetd and a cross-compiler to bootstrap myself. From there, I built a daemon that can convert DRM-free PDFs and ePubs into something Amazon's reader on the Kindle can deal with.
After that, I started to get curious about what else might be possible. It only took a few evenings to get a moderately usable Ubuntu environment running.
Vincent goes on to say the X Windows System took "a bit of hacking" to set up, but "everything else 'just works' with very little configuration." Considering the Kindle's battery life is rated in terms of days, though, we'd wager Ubuntu doesn't quite run at notebook speeds.